1. Field of the Invention
The present invention relates to a plate-type heat pipe suitable for cooling an electric or electronic component, for example, a high heat generating device to be cooled such as a semiconductor chip or the like, and to a method for manufacturing the same.
2. Related Art
An electronic component such as a semiconductor device or the like including a CPU of a personal computer, a laser light emitting diode, a power transistor or the like being mounted on an electric or electronic machine is hard to avoid generation of heat to some extent caused by the use of it, and cooling of the generated heat is becoming a significant technical problem in recent years. As a method for cooling an electric or electronic device needing to be cooled (hereinafter, referred to as a “heat generating device”), there are typically known a method of lowering the air temperature inside the box of a machine by mounting a fan on the machine, a method of directly cooling a heat generating device by mounting a cooling body on the heat generating device, or the like.
As a cooling body to be mounted on a heat generating device, for example a plate material being excellent in heat transfer such as a copper material, an aluminum material or the like, or a plate-type heat pipe or the like are often used.
A plate-type heat pipe is a plate-shaped heat pipe, and may be further called a plane-type heat pipe or a flat-plate type heat pipe. Hereinafter, it is referred to as a plate-type heat pipe.
A heat pipe is briefly described. A heat pipe includes a container having a cavity and has a working fluid sealed in the cavity. The cavity is vacuum-exhausted to make the working fluid easy to evaporate. As a working fluid, water, alcohol, a flon substitute or the like is used in consideration of its adaptivity to a material for the container.
The operation of a heat pipe is briefly described. More specifically, at the heat absorbing side of a heat pipe, a working fluid is vaporized by the heat conducted through a material forming the container of the heat pipe and the vaporized working fluid moves to the heat dissipating side of the heat pipe. At the heat dissipating side, the vaporized working fluid is cooled and returned to a liquid phase.
Then, the working fluid which has come back to the liquid phase flows back to the heat absorbing side. The transfer of the heat is performed by such a phase transformation and movement of the working fluid.
The flowing back of the working fluid is made by gravity or capillary action. In case of the gravity-type heat pipe, the working fluid flows back by disposing the heat absorbing portion lower than the heat dissipating portion. In case of the capillary action-type heat pipe, the working fluid flows back due to a capillary phenomenon of grooves or of a wick by providing the grooves in the inner wall of the container or by inserting the wick comprising a metal mesh, a porous material or the like into the cavity.
Thus, a great amount of heat is transferred in the heat pipe by the phase transformation and movement of the working fluid received inside the hermetically sealed cavity of the heat pipe. Of course, although some amount of heat is transferred by heat conduction through the plate material forming the heat pipe, the amount thereof is relatively small.
To the above-mentioned heat absorbing side a heat generating component is thermally connected. And heat dissipating fins for example are attached to the heat dissipating side. Due to such construction of the heat pipe, a larger portion of the heat from the heat generating component is transferred by the heat pipe and is dissipated through the heat dissipating fins.
Now, the heat pipe has an advantage in which a heat generating component and the heat pipe are easily connected with each other thanks to the features in the shape of the heat pipe. The reason is that by bringing a heat generating component such as a semiconductor device or the like into contact with a main surface of the heat pipe, they can be made contact with each other in a wide area.
For thermal contact of a heat generating component with a heat pipe, a heat transfer grease or a heat transfer rubber may be placed between both of them, or they both may be joined to each other by soldering or the like. In addition, on the opposite face to the main face of the plate-type heat pipe to which the heat generating component is thermally connected, heat dissipating fins or a heat sink and further a fan or the like is preferably mounted. A cooling structure being excellent also in space efficiency can be realized by such an arrangement. In this case, when the main face of the heat pipe to which the heat generating component is thermally connected is deformed, the thermal resistance thereof becomes higher, thus it is necessary for the main face of the heat pipe to have no deformation.
Particularly in recent years, with the increase in integration density and speed of a CPU, the density of heat generation tends to be made higher and the function of the heat pipe must cope with not only requirement for transferring heat but also requirement for such heat diffusion as to diffuse a high-density thermal flux into a low-density thermal flux. And since a printed circuit board with a CPU mounted may be arranged in various positions, a heat pipe also must functions even in a state where it is arranged in various positions. Therefore, it is necessary to prevent occurrence of a so-called dry-out state (i.e., which is a state where the supply of a working fluid lags behind the evaporation thereof at the heat absorbing portion and accordingly the working fluid does not exist in the heat absorbing portion, and the heat transfer operation of the heat pipe cannot be continued). In order to solve the above-mentioned problem, a heat transfer block functioning to diffuse a high-density heat into a low-density heat is provided at the portion in the container to which high density heat generating component is thermally connected, or a wick of a metal mesh, a porous material or the like is properly arranged. A heat transfer block may be inserted as a separate component into the container or may be also formed into one body together with the container.
As a method of forming the container of the plate-type heat pipe, there have been proposed a method of joining two plate-shaped members formed by a press or the like by means of brazing or the like (Japanese Patent Application No.Hei 8-312,980 for example), a method of sealing and joining by folding (Japanese Patent Application No.Hei 10-099,781) or the like.
However, when brazing or soldering is used for joining members forming the hermetically sealed container, a problem occurs that the heat pipe is liable to be deteriorated in performance, depending upon the adaptivity of a brazing material to a working fluid or upon the kind of flux. In order to improve the adaptivity of a brazing material to a working fluid, it is desirable to use a brazing material having a composition approximate to the parent material, but in this case, since the melting point of the brazing material is necessarily made close to the melting point of the parent material, a problem occurs that the strength is lowered by annealing of the parent material. Furthermore, the increase in cost caused by a time required for processing or the use of other materials such as a brazing material or the like cannot be avoided.
On the other hand, in case of sealing by folding, there is a problem that the outer configuration of the plate material is restricted and only simple configurations are applicable. In addition, since this method makes difficult the metal bonding of the members with each other, it is not possible to keep the hermetical sealing for a long period of time. More specifically, there is a problem that a long-term reliability as the heat pipe is deteriorated. Therefore, it is difficult to manufacture a highly hermetically sealed container for the heat pipe by means of the above mentioned method per se.